Say you're Marissa Mayer and your diagnosis is that Yahoo would benefit from some staff reductions. But you also feel that kicking off your tenure with a new round of layoffs would be bad for morale. You think you have some reason to believe that the full-time workers on the Yahoo staff who work from home are a disproportionately low-productivity cohort. Announcing this new policy becomes a way of reducing the size of the staff without throwing everyone into a panic about layoffs and cost-cutting and oh-my-god-should-I-be-looking-for-a-new-job-elsewhere. It's just the new boss putting her stamp on the company.